How to shoot scene with stage performance
 

We have a shoot coming up in January that is by far the most complicated thing we've ever attempted, both logistically because we'll need a bunch of extras and lights, and technically. I'd appreciate some advice on how to pull this off, at least the audio aspect of it.

Here's the scene:
The lead actress performs a song (of her own composition) on stage, accompanying herself on the acoustic guitar. The lead actor walks in, she notices him, and he sits down at a table where he is eventually joined by another woman. I need reaction shots from a several different angles of the woman with the guitar.

The reason this is so tricky is because I want to shoot it with only one camera, capturing a variety of different angles, including a steadicam shot, and have the song be heard constantly in the background. I'm having a hard time figuring out how we can get the close ups of the singer, since she'll obviously have to perform the song several times so that we can change angles. Since she's accompanying herself, it seems unlikely that she'll be able to maintain a perfect tempo, so when I cut to the closeups it's probably not going to match.

What's the best way to handle this? Are there any other considerations that haven't occurred to me yet? I really need to have this worked out to the last detail or it's going to be a major trainwreck. The shoot is two months off and I'm already nervous about it.

How about shooting the closeup first and recording the performance. At the same time, split the sound off to a separate recording. For the long shots and reaction shots, playback the recorded sound from the closeup and have the actress lipsync to it. For that matter you could record the entire number in a sound studio for maximum acoustic quality and shoot all the video to playback on the set in multiple takes, the traditional Hollywood production number technique.

I shoot our operas on two nights with wide shots the first night and close shots the second. At first I thought it would be impossible, but with just a little practice I learned how to quickly cut the audio from each performance seamlessly. I think you should be able to use the different audio tracks from each shot if you edit this properly. Try not to change anything in the audio levels or setup between takes. Then the trick is finding WHERE to make the cuts so they aren't blatant. Don't align the audio cuts with the video cuts, stagger them a little. If needed you can crossfade the two audio tracks (over a duration of maybe 10 frames) instead of making straight cuts.

Well anyway, this is what I do and you really can't tell where one audio clip ends and the next begins. Also a good way to put together the best pieces of each take into one "performance"....

I agree with Steve, the standard procedure is to create the maximum quality audio performance track ahead of time. Then use a CD, or other very stable source, to play back this track for everyone involved in the scene.
You can use the whole track for important master shots and then segment it down (with some lead-in) for parts that you know you'll shoot in specific short bits.
If your camera can take external timecode and your playback source can output timecode, that will save some time in editing but it's not a necessity to have this.
If your camera can't accept external timecode but your NLE can make use of timecode recorded on an audio track, then that's also an option.
You can also shoot a visible timecode display at the head of each shot. That will help you with basic alignment, but isn't much benefit when you're cutting down to very short bits deep into particular footage.
Most importantly you must record guide audio on all cameras. That's the most basic method of achieving sync in post and is used on countless low-budget projects all the time.
You also have to provide a loud enough and clean enough playback signal for everyone to hear it clearly, especially the main performer who will need to play and sing at full volume for a believable performance. Plus there may be some distance involved since the playback system won't be visible in the shot unless you want a stage monitoring system to be part of the look of the scene.
While it doesn't take a rocket-scientist to trigger playback, it really is helpful to have a dedicated, dependable person that handles starting and stopping on command and is familiar with which bit you want to play.
Luckily this isn't a live performance for an audience, otherwise you'd have to do what Boyd does.

One other detail...
 

One thing that I forgot to mention. If you also plan on picking up dialogue from the man and woman seated at the table, that will have to be done with the singing and music silenced, otherwise the playback bleed will be noticeably different than the clean track layed in during post.
If the singer appears in the background, she'll have to mime that segment but you'll still need the timing to be correct. In-ear monitors can work for this since she won't be clearly visible.

Marco,

Your post is a bit unclear as to whether this has to be accomplished in the course of a 'live' performance or not. You say she is 'Onstage' accompanying herself, but you also you will shoot it with a number of angles including a steadycam shot... (Onstage?)

So is it a situation where you can controll playback, and break the shots and action? (think 'music video') or is it more like Boyd's example, where you have to catch the live action over the course of a number of evening's performance?

This reminds me of the documentary on the Blues Brothers DVD. They lip-synced all the audio for that movie with one exception. James Brown was completely incapable of doing anything the same way more than once. They finally gave up, and had to record his performance live! Hopefully you won't have that kind of problem :-)

Well, I was hoping to retain the feel of a live performance, but after reading everybody's advice, it seems to make the most sense to just record the song in advance and play it back while we shoot so she can lip sync. I suppose we can turn the music off while we capture the dialog at the table.

No need to turn it off. Stage your shot so it is aligned with the singer's off-camera ear hidden. Give her a small earbud for that one ear and feed the playback to her through it. The set is silent as far as the dialog recording is concerned and yet she can hear the music enough to play to it in good lipsync. The audience's attention will be focussed on the dialog in front and if her lipsync goes slightly wonky no one is likely to notice.